In sheet-fed offset printing presses, the printed sheets are conveyed through individual printing units over cylinders and drums and, after the last printing unit, through one or more varnishing and other finishing devices. The sheets are transported from a last drum to the delivery stack by chain systems. In order to prevent smearing of the freshly printed sheets during transport between the printing units, between the last printing unit and the delivery stack or the varnishing devices, dryer devices are provided at a various locations along the path of travel.
During the printing of UV inks, UV radiant heaters and, in particular, excimer radiant heaters are used as dryers. The intensity and distribution of UV radiation aimed at the printed side of the sheets is designed such that complete curing of the inks is achieved even at maximum printing speed. Incomplete curing of the ink can cause sheet damage and rejection in the event of contact between the sheet and sheet guide devices or the delivery stack. Excimer radiant heaters for sheet-fed offset presses are known, for example, from EP 378 826 A2, DE 198 57 984 A1 and EP 891 525 B1.
The radiation produced in excimer radiant heaters which have a tubular design is highly divergent. The reason for this is that the discharge chamber has a circular cross section, formed by inner and outer tubes located in the space that accommodates the dielectric. While flat radiant heaters with rectangular reflector irradiation outputs are known, they are useful only at close spacing to the passing steel material, such as at a spacing of a few millimetres. The UV intensity, however, decreases sharply with increasing distance for the sheet material.
In the case of sheet-fed offset presses, the distance between radiant heaters and the printing-material plane is in the range of a few centimeters due to technical reasons and the make-up of the press. In addition, dryer installation space often is limited. In order to achieve useful radiation intensities, the radiation has to be directed distance on to the passing sheets.
The UV radiant heater according to EP 891 525 B1 has a tube extending over the format width of the machine as a radiation generator, which interacts with a reflector arranged in a housing. The radiation generated by the tube is deflected onto the printed material in a directed manner. In high-speed sheet-fed presses that print with UV ink, a high radiant output must be achieved in the region of the printed sheet irradiated by the excimer radiant heater. For the complete curing of the printing inks, a plurality of individual radiant heaters often are arranged one after another if there is appropriate installation space in the printing press.